The use of chemical agents, particularly halides, for the recovery of gold and silver is well known. It was noted very early that the adjunction of sodium chloride to mercury improved the performances of the amalgamation process. This discovery translated into the Patio or Cazo processes, which were implemented on an empirical basis from the early 1600's in Central and South America more than 150 years before the discovery of elemental chlorine by Scheele in 1774. The Patio method involved the digestion of a finely divided gold ore with mercury and sodium chloride, in the presence of air and moisture over a three month period. The values were then collected by further leaching with mercury, followed by amalgam distillation (Egleston, 1887).
Later, in the late 1700s, chloridizing roasting followed by barrel amalgamation was developed in Central Europe as an improved method for gaining access to precious metals from sulfide ores. This process called upon a high temperature treatment of the gold/silver ores in the presence of sodium chloride, air and steam, in order to transform the precious metal sulfides into their corresponding chlorides. The gold and silver was then recovered either by amalgamation or cementation on pure copper (Varley et al, 1923). However, it was discovered that the high temperature chloridizing of gold or silver ores resulted in very important losses of values by volatilization. In some cases these losses reached 80% or more of the precious metal content (Christy, 1888).
It appeared that the presence of pyrites or iron sulfides contributed significantly to the volatilization of gold and silver during high temperature chloridization with NaCl (Croasdale 1903). It was finally established that the mechanism explaining these losses involves the formation of a mixed chloride of gold and iron (AuCl3.FeCl3), which is highly volatile at chloridization temperatures (Eisele et al.).
Elemental chlorine dissolved in water, introduced by Plattner around 1850, constituted an alternative to high temperature chloridization. However, this process was characterized by low efficiency.
The general characteristics of the various processes involving chlorine, either as elemental chlorine or as chlorides, either at ambient temperatures or at high temperatures, were not attractive. The yields obtained with these processes were generally low (often below 50%) and the values were collected as amalgams or as cemented products on copper or iron. In addition, complex procedures were involved in order to obtain the precious metals in a pure form. The environmental impacts of such operations, where large amounts of sulfur are disposed with the tailings, would have been completely unacceptable by current standards.
The advent of cyanide extraction in 1916, terminated the extraction of gold by various forms of chloridation. The cyanide process calls upon the action of a cyanide salt such as sodium cyanide on gold in the presence of oxygen, to give a soluble gold salt (Eq. I):2Au+4NaCN+½O2+H2O→2Na[Au(CN)2]+2NaOH  (Eq. I)
The gold can then be recovered from the cyanide complex by the action of excess zinc (Eq. II):2Na[Au(CN)2]+Zn(excess)→Na2[Zn(CN)4]+2Au  (Eq. II)
Under the best circumstances, gold recovery can be as high as 98%. This process calls for a contact time of one to three days at near ambient temperature in the presence of air.
In some instances the cyanide process performs very poorly. Ores refractory to cyanide extraction can be grouped under the general term of polymetallic ores. In such ores, one finds small amounts of base metals such as copper or zinc, typically 0.1% Cu or 0.3% Zn. Such small amounts qualify the ore as of very low grade for the production of copper or zinc. If such a polymetallic ore body contains some gold (for example, 4 g/T Au or Ag or a mixture of both), the cyanide extraction process does not perform well. The poor performance is due to the base metals, either copper or zinc, (as well as silver), having a much stronger ability to form complexes with cyanide than gold. In fact, this inherent property is used to recover gold from a pregnant solution by zinc treatment following cyanide extraction (see Eq. II). The base metals will consume all the cyanide present and the gold extraction will only begin after all the available base metals, as well as silver, have been dissolved. Because of the excessive consumption of relatively costly cyanide, this process for recovering gold is uneconomical.
Polymetallic ores constitute complex mixtures of sulfides. The tailings discarded as a result of gold and silver extraction using the cyanide process, as well as by other methods, still contain very substantial amounts of sulfur. This sulfur is prone to bio-oxidation (Thiobacillus ferrooxidans), and the resulting drainage is quite acidic and toxic due to its metallic content.
The spent cyanide solutions, kept in large ponds following gold recovery, represents a substantial environmental hazard and has recently created major disasters in Guyana and Central Europe, thus restricting the use of the cyanide process in many areas.
In the last twenty years, chloridation has been reconsidered as a process for extracting base metals such as copper, nickel or silver. The Intec Base Metal Process (Moyes and Houllis, 2002) constitutes a typical example. This process calls for the digestion at 85° C., over a period ranging from 12 to 14 hours, of the sulfides of copper or zinc in a concentrated brine solution (250 g/l NaCl) comprising a cupric mixed halide (BrCl2)Cu prepared electrolytically. The mixture is aerated and the copper is collected as cuprous chloride. The cuprous chloride is decomposed at the cathode to elemental copper by electrolysis upon regeneration of the mixed halide of copper (Eq. III):2CuFeS2+5BrCl2−→2Cu+2+2Fe+3+4S°+5Br−+10Cl−  (Eq. III)
The above described chloridation process was reported as also extracting gold, if present. However, the requirement of recycling copper so as to have the cupric/cuprous system needed to oxidize iron and sulfur, makes this approach very cumbersome when the main concern is gold recovery rather than copper recovery. Further, the electrolytical oxidation of sulfur via the cupric salt, which is regenerated by electrolysis, is a very costly process rendering the treatment of a gold ore having a modest gold content uneconomical. Finally, the presence of elemental sulfur in the tailings is a potential source of acid drainage.
Another chloridation process called Platsol, was reported as being very efficient for the recovery of base and precious metals from sulfide ores (Ferron et al, 2002). This process involves a pressure oxidation in the presence of oxygen and sulfuric acid in an autoclave at a temperature above 200° C. The implementation of such a technique is very capital-incentive, calling for titanium autoclaves and a source of pure oxygen. The operation of this equipment is also prone to problems due to scaling of the reactor, complicating heat transfer. The sulfur resulting from the operation is in an innocuous form, i.e. a hydrated iron sulfate jarosite). The high capital and operating costs render this approach unattractive for polymetallic sulfides having a modest gold content.
Other techniques such as the Plint process (Frias et al, 2002) or, the Ito process (Kappes et al, 2002), are techniques used for the recovery of gold and silver from sulfides, by oxidation with ferric chloride in concentrated brine. The ferrous chloride is re-oxidized to ferric chloride by chlorine alone or by exposure to air and hydrochloric acid (Eq. IV):2PbS.Ag2S.3Sb2S3+24FeCl3→24FeCl2+2PbCl2+2AgCl+6SbCl3+12S°  (Eq. IV)
In these processes, sulfur is again oxidized electrochemically via the oxidation of ferrous chloride by chlorine or HCl. As explained previously, such an approach is costly for the recovery of gold or silver from sulfide ores, because of the electrochemistry involved. Elemental sulfur is again discarded with the tailings, generating a potential source of acid drainage.
There thus remains a need for an improved method for the recovery of gold and silver from polymetallic ores.
The present invention seeks to meet these and other needs.